Apparatus and method for preventing an electrical backfeed

ABSTRACT

At least one aspect is directed to a transfer switch including a first input to couple a first multiphase low voltage electrical supply to the transfer switch, and a second input to couple a second multiphase low voltage electrical supply to the transfer switch. The transfer switch also includes a first set of switches in electrical communication with the first input and a second set of switches in electrical communication with the second input. A control module monitors and controls operation of both the first set of switches and the second set of switches to detect a malfunction of any switches included in at least one of the first set of switches and the second set of switches and prevent the first input from being placed in electrical communication with the second input.

BACKGROUND OF INVENTION

1. Field of Invention

Embodiments of the invention relate generally to transfer switches. Morespecifically, at least one embodiment relates to an apparatus and methodfor preventing an electrical backfeed resulting, for example, fromoperation of an automatic transfer switch.

2. Discussion of Related Art

Transfer switches are employed to increase the reliability of anelectrical supply to a load by allowing the load to be supplied from twoor more sources. For example, a utility electrical supply (e.g., apublic utility, a municipal utility, etc.) may provide one electricalsupply to a load and a source of backup power (e.g., a standby/emergencygenerator, uninterruptible power supply, etc.) can provide a secondelectrical supply to the same load. The transfer switch is used totransfer the load between the two electrical supplies in the event thatone of them is unavailable. With some exceptions, applicable electricalcodes and national manufacturing standards generally require that thetransfer switch maintain electrical isolation between the two electricalsupplies for operational and safety reasons. That is, the conductors ofthe first electrical supply and the conductors of the second electricalsupply cannot be connected to one another, even momentarily, e.g., thetwo electrical supplies cannot be connected to the load in parallel. Abackfeed is created when phase conductors of the first electrical supplyand phase conductors of the second electrical supply are connected toone another, for example, as a result of a switch failure in a transferswitch. In one failure mode, contacts in the transfer switch weld shut,that is separate contact surfaces fuse together. Contacts do weld shutand fail to operate due to arcing and/or overheating. If for example, acontact connected to the utility fails to open in the transfer switch,the generator supply will backfeed electricity to the utility when thegenerator is connected to the load.

To meet the requirements for electrical isolation of differentelectrical supplies, transfer switches (in particular, automatictransfer switches) often employ contactors, force guided relays ormotorized circuit breakers to perform the switching that transfers theload from one electrical supply to another. The contactors, relays orcircuit breakers are mechanically interlocked to prevent a backfeedbetween the various power sources connected to the transfer switch.These approaches generally result in transfer switches that are moreexpensive and more complex than practical for residential applications.

In another approach, control logic is used with power transfer relays inan automatic transfer switch in a residential installation. Regardlessof the status of relay contacts, the control logic initiates a transferof a load to a generator electrical supply when the logic detects a lossof voltage in a utility electrical supply.

Manual transfer switches are sometimes used as an alternative toautomatic transfer switches to connect one of two electrical supplies tothe load. Manual transfer switches suffer from the obvious drawback thathuman intervention is required to switch from one electrical supply toanother electrical supply. In addition, these switches typically includea mechanical interlock to prevent different electrical supplies frombeing connected to one another.

SUMMARY OF INVENTION

In order to prevent a backfeed between two or more electrical suppliesused to supply a load, at least one embodiment of the invention detectswhen a switch in an transfer switch has malfunctioned.

In one aspect of the invention, a transfer switch includes a first inputto couple a first multiphase low voltage electrical supply to thetransfer switch and a second input to couple a second multiphase lowvoltage electrical supply to the transfer switch. The transfer switchalso includes a first set of switches in electrical communication withthe first input, a second set of switches in electrical communicationwith the second input, and a control module. The control module monitorsand controls operation of both the first set of switches and the secondset of switches to detect a malfunction of any switches included in atleast one of the first set of switches and the second set of switchesand prevent the first input from being placed in electricalcommunication with the second input.

In one embodiment, the output couples the transfer switch to a load, anda third set of switches selectively couples the first multiphase lowvoltage electrical supply and the second low voltage electrical supplyto the output. The control module monitors and controls operation of thethird set of switches to detect a malfunction of any switches includedin the third set of switches and prevent the first input from beingplaced in electrical communication with the second input. In a versionof this embodiment, the transfer switch includes a switching module witha switch included in the first set of switches, a switch included in thesecond set of switches, and a switch included in the third set ofswitches.

In another one embodiment, the first input is adapted to couple to afirst split-phase electrical supply and the second input is adapted tocouple to a second split-phase electrical supply.

In another aspect, the invention provides a method of preventing abackfeed through a transfer switch. The transfer switch includes a firstinput adapted to receive a first multiphase low voltage power source, asecond input adapted to receive a second multiphase low voltage powersource and an output. The first input is connected to the output. Atransfer is initiated to disconnect the output from the first input andconnect the output to the second input. A plurality of switches areoperated to complete the transfer. At least one of the plurality ofswitches is monitored to detect a malfunction of any of the switches,and to prevent the first input from being placed in electricalcommunication with the second input, the transfer is stopped if amalfunction is detected.

In a further aspect of the invention, a transfer switch includes a firstinput to couple a first multiphase low voltage electrical supply to thetransfer switch and a second input to couple a second multiphase lowvoltage electrical supply to the transfer switch. The transfer switchalso includes a first set of switches in electrical communication withthe first input, a second set of switches in electrical communicationwith the second input, and means for detecting a malfunction in any ofthe first set of switches and the second set of switches. Upon detectinga malfunction, the means for detecting the malfunction in any of thefirst set of switches and the second set of switches prevents aconnection of at least one of the first input and the second input.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, are not intended to be drawn to scale. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a block diagram of a transfer switch employed in an electricalsystem according to one embodiment of the invention;

FIG. 2 is schematic diagram of switch connections in a transfer switchaccording to one embodiment of the invention;

FIG. 3 is a flow chart of a process employed with one embodiment of thetransfer switch of FIG. 2 beginning at a point in time at which neitherprimary nor alternate power is available;

FIG. 4 is a flow chart of another process employed with one embodimentof the transfer switch of FIG. 2 beginning at a point in time when aprimary source of power is supplying a load;

FIG. 5 is a flow chart of a further process employed with one embodimentof the transfer switch of FIG. 2 beginning at a point in time when analternate source of power is supplying a load;

FIG. 6 is a schematic diagram of switch connections in anotherembodiment of a transfer switch according to the invention;

FIG. 7 is a schematic diagram of switch connections in a furtherembodiment of a transfer switch according to the invention; and

FIG. 8 is a schematic diagram of switch connections in yet anotherembodiment of a transfer switch according to the invention.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” or “having,”“containing”, “involving”, and variations thereof herein, is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items.

Generally, a transfer switch used in a residential application isolatesseparate power sources when transferring the residential load from aprimary power source to an alternate power source. In one embodiment,the invention facilitates operation in this manner by sensing whetherthe switches employed in the transfer switch are operational, forexample, by sensing whether the switches are in positions that areconsistent with their states as determined by control logic. In aversion of this embodiment the transfer switch is an automatic transferswitch.

Referring now to FIG. 1, one embodiment of a transfer switch 20 is shownconnected in an electrical system 22. In addition to the transfer switch20, the electrical system 22 shown in FIG. 1 includes a circuit breakerpanel 24, an uninterruptible power supply 26 and an emergency generator28. The circuit breaker panel 24 is connected to a primary source ofpower 30, for example, a utility supply. Each of the uninterruptiblepower supply 26 and the emergency generator 28 provide an alternatesource of power to the transfer switch 20. The emergency generator 28can be driven by any of a variety of power plants including a gasengine, a diesel engine, gas turbine, etc. Although the uninterruptiblepower supply 26 and the emergency generator 28 are shown, otheralternative sources of power could be used to provide backup powerincluding a second utility supply, a fuel cell inverter and alternativeenergy sources such as wind turbines, and solar panels.

In FIG. 1, the primary source of power 30 is a multiphase power source.As used herein, the term multiphase describes any power source thatincludes at least two line conductors. In one embodiment, the powersupplied by each line conductor is out of phase relative to the otherline conductors. Thus, as used herein, a multiphase power source caninclude two or more phase conductors from a three phase system. Inaddition, a multiphase power source can include two line conductors froma split-phase power source. In one embodiment, the primary source ofpower 30 is a 120/240 volt split-phase power source.

The circuit breaker panel 24 includes a main circuit breaker 32,two-pole circuit breakers 34, single-pole circuit breakers 36, a neutralbus 37 and a ground bus 38. In one embodiment, the main circuit breaker32 is a 200 ampere circuit breaker, the two-pole circuit breakers are 20ampere circuit breakers, and the single pole circuit breakers are 15ampere circuit breakers. In one embodiment, two-pole circuit breakers 34include two circuit breakers that are mechanically connected to insurethat they operate substantially simultaneously. In one embodiment, thecircuit breaker panel 24 is installed in a residential electricalservice. In a version of this embodiment, each of the single-polecircuit breakers 36 are connected to a single phase input 40 of thetransfer switch 20, and each of the two-pole circuit breakers areconnected to multiphase input 42. In an alternate embodiment, onlyselected single-pole circuit breakers 36 are connected to a single phaseinput 40 and only selected two-pole circuit breakers 34 are connected toa multiphase input 42. A first alternate source of power is connected toa first alternate input 44 and a second alternate source of power isconnected to a second alternate input 46. Where an uninterruptible powersupply 26 is used, the transfer switch 20 may include a UPS output 48 tosupply power to the uninterruptible power supply 26, for example, tocharge the UPS batteries.

Electrical load circuits (e.g., supplying power to receptacles, lightingcircuits, etc.) are connected to outputs of the transfer switch. Eachmultiphase load circuit 49 is connected to a separate multiphase output,for example, multiphase output 50. Each single phase load circuit 51,51′ is connected to a separate single-phase 120 volt output, forexample, single-phase outputs 52, 52′, respectively. In one embodiment,each single-phase output supplies 120 VAC and each multiphase outputsupplies 240 VAC.

The transfer switch 20 includes multiphase switching modules 54, 54′,and single-phase switching modules 56, 56′. In the embodiment shown inFIG. 1, each multiphase switching module 54, 54′ switches one lineconductor of a multiphase circuit, and each single-phase switchingmodule 56, 56′ switches the single line conductor of a single-phasecircuit. In a version of this embodiment, a single pair of multiphaseswitching modules dedicated for multiphase operation are included in thetransfer switch 20. In another embodiment, the switching modules arereconfigurable. For example, the transfer switch 20 can be equipped withtwo switching modules 56, 56′ that can separately switch a lineconductor of a single-phase circuit and can be reconfigured to operateas a multiphase switching module 54, 54′ to switch a line conductor of amultiphase circuit. In a version of this embodiment, the transfer switchis equipped with a plurality of reconfigurable switching modules thatcan be reconfigured to operate as either single-phase switching modules56, 56′ or multiphase switching modules 54, 54′. That is, the quantityof each type of switching module can be changed to suit the requirementsof a specific installation.

The transfer switch 20 also includes a main control module 58, amultiphase control module 59, a user interface 60, a power supply 62, acommunications module 64, a UPS switch 66 and a neutral bus 68. Eachswitching module 54, 54′, 56 and 56′ can include current and voltagesensing with outputs communicated to the main control module 58, forexample, to monitor for energy use, overload conditions, etc. In oneembodiment, logic in the multiphase control module 59 senses theposition of switches included in the multiphase switching modules 54,54′ and controls the operation of the switches in the multiphaseswitching modules 54, 54′. In a version of this embodiment, a singlemultiphase control module 59 controls each multiphase switching module54, 54′ employed in the transfer switch 20. In another embodiment, themain control module 58 includes logic to sense the position of one ormore switches included in both the multiphase switching modules 54, 54′,and the single-phase switching modules 56, 56′, and logic to control theoperation of the switches included in both the multiphase switchingmodules 54, 54′ and the single-phase switching modules 56, 56′.

The main control module 58 can also control operation of the UPS switch66. For example, in one embodiment, power is supplied to the UPS input48 from the circuit breaker panel 24 during normal operation, however,when power from the normal supply 30 is lost, the main control module 58switches the position of UPS switch 66 so that power is supplied to theUPS from an alternate source, i.e., the UPS output 48 is connected tothe second alternate input 46, for example, a backup generator.

In one embodiment, the user interface 60 includes a display and an inputdevice (e.g., a keypad, a touch screen, etc.) to communicate with themain control module 58. The user interface provides a user theopportunity to review the status of the transfer switch 20 including: 1)monitoring the existing electrical parameters (e.g., current, voltage,etc.) for one or more of the load circuits 49, 51, 51′; 2) monitoringstored electrical parameters (e.g., power consumption, peak current,etc.); and 3) adjusting set points, for example, a minimum voltage levelthat triggers a transfer from primary power to one of the alternatepower sources. In addition, the user interface 60 can provide a means ofconfiguring the transfer switch 20 to meet the user's needs. Forexample, the user interface 60 can allow the user to establish whetherreconfigurable switching modules are deployed as single-phase switchingmodules 56, 56′ or multiphase switching modules 54, 54′. The user mayalso be able to select the ampacity of the load circuits 49, 51, and51′, for example, by choosing from standard ratings such as 15 ampereand 20 ampere ratings. The user may also employ the user interface 60 toadjust setpoints or delays. As one example, a user may establish asetpoint for initiating a generator start signal to start a backupgenerator after primary power is lost.

The transfer switch 20 also includes a power supply module 62 that caninclude one or more circuits for generating control power and logiclevels used in the switch 20. In one embodiment, an input to the powersupply module 62 is supplied from one of the primary source and thealternate source. Generally, in one embodiment, the switching of thepower supply input is controlled either manually or by logic thatdetermines the availability of primary and alternate power and thestatus of at least the multiphase switching modules (e.g., determineswhich source is supplying the load circuits). In a version of thisembodiment, the power supply input is connected to the primary sourcewhenever the primary source is available. When the primary source isunavailable, the power supply input is automatically disconnected fromthe primary source and is automatically connected to the alternatesource if the alternate source is available. Further, in one version thepower supply input is automatically disconnected from the alternatesource and automatically re-connected to the primary source when theprimary source again becomes available, i.e., primary source power isrestored. In one embodiment, the power supply module 62 provides +3.3 V,+5 V, +12V and −12V DC outputs.

The communications module 64 provides a connection for an externalcommunication network, for example, a connection to one or both of alocal area network or a wide area network, e.g., a wireless network. Asa result, a remote user can access the user interface 60 via phone, aremote touchpad, or a remote computer.

Referring now to FIG. 2, one embodiment of multiphase switching modules70, 70′, which may be used in the transfer switch 20 in place of themultiphase switching modules 54, 54′ of FIG. 1, are shown. In theembodiment shown in FIG. 2, the multiphase switching module 70 providesswitching for a first line conductor (e.g., a first phase) of amultiphase supply and multiphase switching module 70′ provides switchingfor a second line conductor of the multiphase supply (e.g., a secondphase). Together, the multiphase switching modules 70, 70′ include threesets of switches: a first set of switches includes switch 72 and switch74; a second set of switches includes switch 76 and switch 78; and, athird set of switches includes switch 80 and switch 82. In theembodiment shown in FIG. 2, each switch is a single pole double throwswitch, that is, there is a single switch element that can be movedbetween a first closed position and a second closed position (e.g., aform C relay). In addition each switch 72, 74, 76, 78, 80 and 82 iselectrically operated such that each switch has a normally open contactposition (NO) and a normally closed contact position (NC). Although eachswitch 72, 74, 76, 78, 80, 82 is described as being included in a singlepole double throw relay in the embodiment of FIG. 2, each switch can beany contact capable of carrying current and operable between a firstposition and a second position. As one example, each switch 72, 74, 76,78, 80, 82 can also be replaced by two single pole single throw switchesthat are operated in a manner that mimics the operation of thecorresponding single pole double throw switches. In one embodiment,switches 72, 74, 76, 78, 80 and 82 are a miniature relay for printedcircuit board mounting, for example, part number 832A-1C-S-24DCmanufactured by Song Chuan. In other embodiments, switches 72, 74, 76,78, 80, 82 could instead be included in contactors, circuit breakers, orsolid state switches. Further, in one embodiment, switches 72, 74, 76,78, 80, 82 are included in a single multiphase switching module.

In FIG. 2, the multiphase switching modules 70, 70′ provide a primaryinput 91 including a first terminal 90 for multiphase switching module70 and a second terminal 92 for multiphase switching module 70′. Themultiphase switching modules 70, 70′ also include an alternate input 93including a first terminal 94 for multiphase switching module 70 and asecond terminal 96 for multiphase switching module 70′. The multiphaseswitching modules 70, 70′ also provide an output 84 including a firstterminal 86 for multiphase switching module 70 and a second terminal 88for multiphase switching module 70′. In one embodiment, a multiphasecontrol module 98 (similar to multiphase control module 59 discussedabove) communicates with the multiphase switching modules 70, 70′ via acommunication bus 100 which is connected to each switching module 70,70′. The multiphase control module controls the operation of theswitches 72, 74, 76, 78, 80, 82 to supply power to the output 84 whilepreventing a connection between the primary input 91 and the alternateinput 93. In one embodiment, the multiphase control module 98 includeslogic that prevents even a momentary connection between the primaryinput 91 and the alternate input 93 including connections via a load,i.e., it prevents a connection between any of the first terminal 90 andthe second terminal 92 of the primary input 91, and any of the firstterminal 94 and the second terminal of the alternate input 93.

The control module 98 can be implemented in hardware, software, firmwareor a combination thereof. In one embodiment, the control module 98 is acomplex programmable logic device (“CPLD”), for example, from the MAX7000 family sold by Altera Corporation. In another embodiment, thecontrol module 98 can be implemented in a microprocessor ormicrocontroller executing embedded software and/or firmwareinstructions. In an alternative embodiment, the control module 98 isimplemented in combinatorial logic and sequential logic.

The communication bus 100 can be any single line or multi-line buscapable of transmitting control and sensing signals between themultiphase control module 98 and the multiphase switching modules 70,70′. For example, in one embodiment, the communication bus 100 includesa plurality of discrete lines with a separate line dedicated to eachinput to the multiphase control module 98 (e.g., voltage sensing inputs)and a separate line dedicated to each output. The outputs can, forexample, be signals supplied by the multiphase control module 98 totransistors co-located with relays employed in multiphase switchingmodule 70, 70′, for example, mounted together on a printed circuitedboard. The signals provided by the multiphase control module 98 canoperate a transistor used to switch on and off a coil of a relayemployed in multiphase switching module 70, 70′. In a furtherembodiment, the communication bus 100 also includes one or more linesthat connect the multiphase control module 98 to a main control module,such as main control module 58 of transfer switch 20. In a version ofthis embodiment, the communication bus also includes one or more linesthat connect the main control module 58 to the multiphase switchingmodules 70, 70′. In addition, the transfer switch may include a controlpower bus (not shown) that is connected to the main control module 58and the multiphase control module 98.

The multiphase switching modules 70, 70′ include a plurality of sensingnodes that can each supply an input to the multiphase control module 98via the communication bus 100. One or more of the sensing nodes can alsosupply an input to the main control module 58 via the communication bus100. In an alternate embodiment, the main control module 58 receives asinputs signals provided by the multiphase control module 98 instead ofor in addition to signals provided from the multiphase switching modules70, 70′. In addition, communication bus 100 connects the control module98 to each switch 72, 74, 76, 78, 80, 82 to provide switching controllogic to the switches. In one embodiment, based on a monitored voltageor current provided by sensing nodes, the multiphase control module 98senses the availability of normal and alternate power sources and thestatus of switches 72, 74, 76, 78, 80, 82, i.e., whether the switch isin the normally-closed or the normally-open position. In thisembodiment, the multiphase control module 98 uses this information todetermine whether the switches must be operated to supply power to theload, for example, when it is necessary to switch to an alternate sourceof power because the primary source of power is unavailable. Inaddition, the multiphase control module 98 implements control logic toinsure that a backfeed, even a momentary backfeed, is not created whenone or more of the switches 72, 74, 76, 78, 80, 82 are operated.

In a further embodiment, the main control module 58 determines theavailability of the primary and alternate power sources and providesinformation to the multiphase control module 98 regarding the powersource that should be used. The multiphase control module 98 is thenresponsible for operating the switches 72, 74, 76, 78, 80, 82 in amanner that prevents a backfeed. In a version of this embodiment, themain control module 58 receives inputs associated with the primary andthe alternate sources of power, for example, from one or more sensingnodes or from another voltage-sense signal that may not be provided bythe switching module 70, 70′, e.g., the voltage sensing may be performedelsewhere in the transfer switch 20 or the circuit breaker panel 24. Themain control module 58 uses the information provided by thevoltage-sense signal to determine the availability of the primary andthe alternate power sources. In this embodiment, the main control module58 determines the appropriate source of power and provides thisinformation as one or more inputs to the multiphase control module 98.The multiphase control module 98 then determines whether any switchingis required, and if so, whether the switching can be accomplishedwithout creating a backfeed.

In one embodiment, the transfer switch 20 complies with Underwriter'sLaboratory Standard 1008 (“UL 1008”). UL 1008 requires that a transferswitch prevent even a momentary backfeed between different sources ofpower (e.g., prevent a backfeed between a primary and an alternatesource of power) even when there is only a single switch failure. In aversion of this embodiment, three switches (e.g., 72, 76, 80) areemployed in each of switching modules 70, 70′. In an alternateembodiment, however, only two switches are employed in each multiphaseswitching module 70, 70. For example, in FIG. 2, switches 76 and 78 areeliminated, or alternatively, switches 72 and 74 are eliminated in thisalternate embodiment.

In one embodiment, transfer switch 20 is configured for operation withprimary and alternate power sources that include 3 or more phases. Forexample, a transfer switch 20 can include multiphase switching modulescapable of switching 3-phase sources in a manner that complies with UL1008. In a version of this embodiment, a third multiphase switchingmodule corresponding to multiphase switching modules 70, 70′ is added tothe transfer switch 20 to switch the third phase of a multiphase source.

In one embodiment, to prevent a connection between a normal source ofpower and an alternate source of power, the multiphase control module 98also determines, based on the outputs of the sensing nodes, whether oneor more of switches 72, 74, 76, 78, 80, 82 are operative beforegenerating a control signal to operate a switch. In the embodiment shownin FIG. 2, these sensing nodes include a first primary sense node 102, asecond primary sense node 104, a first alternate sense node 106, asecond alternate sense node 108, a first primary switched node 110, asecond primary switched node 112, a first alternate switched node 114, asecond alternate switched node 116 and an output node 118. In oneembodiment, an optoisolator 120 is connected across the output 84between terminals 86 and 88 to provide the signal at the output node118. In one embodiment, for example, the optoisolator corresponds topart number SFH615A-4 manufactured by Vishay. In one embodiment, thesensing nodes 102, 104, 106, 108, 110, 112, 114, 116 employ the sameoptoisolator to generate logic signals. In another embodiment, each pairof sensing nodes (e.g., 102/104, 106/108, 110/112, 114/116) can employ aseparate optoisolator so that a single logic signal is provided for theprimary input, the alternate input, etc.

In one embodiment, the multiphase control module 98 of FIG. 2 include aconnection to the system neutral. In a version of this embodiment, theneutral input allows for independent detection of a single-switchfailure. For example, such an approach can provide information used todetermine whether switch 72 or switch 74 failed. In one or more versionsof this embodiment, the neutral conductor is not disconnected whenswitching is performed.

Operation of the multiphase switching modules 70, 70′ of FIG. 2 will nowbe described with reference to the flow charts provided in FIGS. 3, 4and 5. In addition, in the interest of clarity, each sense node isassociated with a corresponding logic point as follows: the firstprimary sense node 102 corresponds with logic point PR1; the secondprimary sense node 104 corresponds to logic point PR2; the firstalternate sense node 106 corresponds to logic point ALT1; the secondalternate sense node 108 corresponds to logic point ALT2; the firstprimary switched node 110 corresponds to logic point SWPR1; the secondprimary switched node 112 corresponds to logic point SWPR2; the firstalternate switched node 114 corresponds to logic point SWALT1; thesecond alternate switched node 116 corresponds to SWALT2; and, theoutput node 118 corresponds to CKTO. One additional logic point, CPLcorresponds to the status of the power supplied to the multiphasecontrol module 98, i.e., the control power. Each logic point typicallyprovides HI and LO logic levels which depend upon the status of thetransfer switch 20.

For purposes of explaining operation of the multiphase switching modules70, 70′ in FIGS. 3, 4 and 5 the following logic scheme applies: each oflogic points PR1, and PR2 provide a logic HI signal when thecorresponding line voltage of the primary source is present at PR1 andPR2, respectively; each of logic points ALT1 and ALT2 provide a logic HIsignal when the corresponding phase voltage of the alternate source ispresent at ALT1 and ALT2, respectively; each of logic points SWPR1,SWPR2, SWALT1, and SWALT2 provide a logic LO signal when a voltage ispresent at their respective sensing node; the CKTO logic point providesa logic LO signal when voltage is present at the output 84; and, the CPLprovides a logic HI signal when voltage is present on a control powerbus for the multiphase control module 98. In other embodiments, thelogic signals corresponding to the circuit conditions mentioned abovediffer from the logic states described above. In one embodiment, an optoisolator is used with each logic point (e.g., opto isolator 120). Inanother embodiment, an opto isolator is only used with those logicpoints that provide a LO signal when voltage is present at thecorresponding sensing node. Of course, with the flexibility provided bydiscrete logic sensing, a variety of configurations may be employed. Forexample, in one embodiment, each logic point provides a logic HI signalwhen a voltage is present at the corresponding sensing node. In analternate embodiment, each logic point provides a logic LO signal when avoltage is present at the corresponding sensing node.

The physical location of the sensing nodes may also be changed invarious embodiments. In one embodiment, the primary sense nodes 102, 104and the alternate sense nodes 106, 108 are not located in multiphaseswitching modules 70, 70′. Instead, the primary sense nodes 102, 104 canbe located on the line side of multiphase switching modules 70, 70′, forexample at primary input 42. The alternate sense nodes 106, 108 can alsobe located on the line side of switching modules 70, 70′, for example,at second alternate input 46. In one embodiment, the sensing nodes 102,104, 106, 108 are connected to inputs of the main control module 58.

FIG. 3 depicts a flow diagram of a process 300 for controlling themultiphase switching modules 70, 70′ in accordance with one embodiment.Specifically, the process 300 begins at a time when neither the primarynor the alternate power source is available and continues until suchtime as power first becomes available on either the primary input 91 orthe alternate input 93, shown in FIG. 2. The CPL logic point provides alogic LO signal when neither the primary nor the alternate power sourcesare available (Stage 310). At this time, each of the switches 72, 74,76, 78, 80, 82 are in their normally closed (NC) position and thecontrol logic is off. When either primary power or alternate powerbecome available, control logic becomes available and CPL transitions toa logic HI state. If primary power becomes available, PR1 provides alogic HI signal because voltage is present at first primary sense node102 and PR2 provides a logic HI signal because voltage is also presentat second primary sense node 104 (Stage 312). If alternate power becomesavailable, ALT1 provides a logic HI signal because voltage is present atfirst alternate sense node 106 and ALT2 provides a logic HI signalbecause voltage is also present at second alternate sense node 108(Stage 314).

Because the default state of the switches 72, 74 and 80, 82 is normallyclosed (NC) in the embodiment shown in FIG. 2, primary power is suppliedto the output 84 of the multiphase switching modules 70, 70′ if primarypower is restored before alternate power is available (Stage 313). Thatis, without operating any switches 72, 74, 76, 78, 80, 82, the primaryinput 91 is connected to the output 84 when neither primary power noralternate power are available. Conversely, the default state of theswitches 76, 78 and 80, 82 disconnects the alternate input 93 from theoutput 84 when neither primary power nor alternate power are available.Thus, in one embodiment, when the alternate power supply is available atthe alternate input 93, the multiphase control module 98 processes logicto determine whether the alternate input 93 should be connected to theoutput 84. If primary power is unavailable, the primary power source isisolated (Stage 315).

FIG. 4 depicts a process 400 including a sequence of stages employed inone embodiment to transfer the load connected to output 84 from theprimary power source to the alternate power source. The process 400starts at Stage 313 of FIG. 3 with the primary power source supplyingpower to the load. In one embodiment, the multiphase control module 98continuously monitors the state of PR1 and PR2 to determine whetherprimary power is available (Stage 416). As described above, PR1 and PR2each provide a logic HI signal if power is present at the primary input91. When PR1 and PR2 are HI the primary input 91 remains connected tothe output 84. In one embodiment, the availability of alternate power atalternate input 93 allows a user to override the standard logic,disconnect the primary input 91, and connect the alternate input 93 tothe output 84, for example, to conduct load testing of the alternatesource. In one embodiment, a transition of either or both of PR1 and PR2to a LO state provides an indication that primary power is unavailable.When primary power is unavailable, the multiphase control module 98determines whether alternate power is available. In one embodiment, themultiphase control module 98 continuously monitors the state of ALT1 andALT2 to determine whether alternate power is available (Stage 418). Theavailability of power at alternate input 93 is indicated when ALT1 andALT2 provide a logic HI signal. For example, in one embodiment, each ofALT1 and ALT2 provide a logic LO signal until a generator connected tothe alternate input 93 starts and begins to supply voltage to thealternate input 93, at which time ALT1 and ALT2 transition to a logicHI. When the primary source is unavailable and alternate power issensed, the multiphase control module 98 provides logic signals to openswitches 72 and 74 in order to disconnect the primary input 91 from theoutput 84 (Stage 420). In one embodiment, the multiphase control module98 also provides a generator start signal to start a generator whenprimary power is unavailable. In another embodiment, the main controlmodule 58 provides the generator start signal.

Once multiphase control module 98 provides the logic signals to openswitches 72, 74, the multiphase control module 98 determines whether theswitches 76, 78 connected to the alternate input are in the correctposition (Stage 422), i.e., normally closed (NC). If either switch 76,78 is in the normally open (NO) position, a voltage is present at thecorresponding sensing node 114, 116, respectively. If switch 76 hasmalfunctioned and remains in the normally open position then thepresence of voltage at sensing node 114 is indicated by logic pointSWALT1 providing a logic LO signal. Similarly, if switch 78 hasmalfunctioned and remains in the normally open position the presence ofvoltage at sensing node 116 is indicated by logic point SWALT2 providinga logic LO signal. If either SWALT1 or SWALT2 is LO, the multiphasecontrol module 98 is not provided a logic signal to operate switches 76and 78, but instead generates a signal indicating that there is a faultwith either or both of switches 76, 78 (Stage 424). In one embodiment, aswitch-failure indication is provided at the user interface 60 (e.g., anaudible alarm, a flashing display, etc.) and an alarm signal istransmitted to a remote location via the communication module 64 toindicate that there is a problem with transfer switch 20. Further,although the fault sensing described herein is referred to as detectinga switch failure or switch malfunction, the fault sensing detects anyfailure mode that prevents contacts from operating according to theswitching logic. For example, the multiphase control module 98 willdetect a failure due to an open relay coil, mechanical binding of aswitch operator, and the like. Thus, switch failure and switchmalfunction refer to the failure of a contact to be in the state desiredby the switching logic regardless of the cause.

If each of SWALT1 and SWALT2 is HI, the multiphase control module 98provides logic signals to operate switches 76, 78 (Stage 426). Themultiphase control module 98 then determines whether the switches 76, 78connected to the alternate input have moved to the normally open (NO)position in response to the logic signals provided by the control module98 (Stage 428). At this time, the presence of voltage (supplied from thealternate input 93) at sensing nodes 114 and 116 is expected. Therefore,if both SWALT1 and SWALT2 are LO, the switches 76, 78 are in the correctpositions. Conversely, a logic HI signal provided by SWALT1 indicatesthat switch 76 remains in the normally closed (NC) position, and a logicHI signal provided by SWALT2 indicates that switch 78 remains in thenormally closed (NC) position. If either SWALT1 or SWALT2 is in a logicHI state, the alternate input 93 is not connected to the output 84.Instead, the multiphase control module 98 generates a signal indicatingthat there is fault with either or both of switches 76, 78 (Stage 424).

If each of SWALT1 and SWALT2 is LO, the multiphase control module 98provides logic signals to operate switches 80, 82 (Stage 430). In theembodiment shown in FIG. 2, switches 80, 82 are source-selector switchesthat when in their normally closed (NC) position connect the output 84to the switches 72, 74 connected to the primary input 91. When in theirnormally open (NO) position, switches 80, 82 connect the output 84 tothe switches 76, 78 connected to the alternate input 93. Thus, providedthat switches 76, 78 are in their normally open (NO) position, thealternate input 93 is connected to the output 84 when switches 80, 82move to their normally open (NO) position in response to logic signalsprovided by the multiphase control module 98 at Stage 430. In addition,the multiphase control module 98 confirms whether the switches 80, 82have in fact moved to their normally open (NO) position in response tothe logic signals. In the embodiment shown in FIG. 4, sensing node 118provides an indication of the position of switches 80, 82 (Stage 432).If the CKTO logic point provides a logic HI signal the multiphasecontrol module 98 generates a signal indicating that there is fault witheither or both of switches 80, 82 (Stage 424). In one embodiment, inresponse to a fault associated with switches 80, 82, the multiphasecontrol module generates logic signals to operate switches 76, 78 inorder to isolate the alternate input 93 from the faulty switch orswitches 80, 82. If the CKTO provides a logic LO signal, the alternateinput 93 remains connected to the output 84 until the primary powerbecomes available again at primary input 91 (Stage 433).

Referring now to FIG. 5, a flow diagram depicts a process 500 employedin one embodiment to transfer the load connected to the output 84 fromthe alternate power source to the primary power source. Initially, thealternate input 93 is connected to the output 84 in order to supply theload during periods when the primary power source is unavailable (Stage534). In one embodiment, during the period that the primary power sourceis unavailable, the multiphase control module 98 continuously monitorsfor the return of the primary power at sensing nodes 102, 104 (Stage536). When primary power is again available, both PR1 and PR2 provide alogic HI signal. In response, the multiphase control module 98 performsthe switching necessary to isolate the alternate input 93 from theoutput 84 and to connect the primary input 91 to output 84. Themultiphase control module 98 generates logic signals to operate switches76, 78 so that each switch 76, 78 moves to the normally closed (NC)position (Stage 538). The multiphase control module 98 senses the statusof each switch 76, 78 based on the state of logic points SWALT1 andSWALT2 corresponding to sensing nodes 114, 116, respectively (Stage540). A voltage present on sensing nodes 114, 116 indicates that thecorresponding switch (76, 78, respectively) has malfunctioned andremains in the normally open (NO) position. Thus, if either or both ofSWALT1 and SWALT2 are LO the multiphase control module 98 senses amalfunction of the corresponding switch (76, 78, respectively) andgenerates a signal (e.g., a fault signal, trouble signal, etc.)indicating that there is fault with either or both of switches 76, 78(Stage 424). In one embodiment, a generator supplying the alternatesource is shutdown regardless of the status of switches 76, 78 when theprimary source is again present at the primary input 91. In oneembodiment, once the multiphase control module 98 generates a faultsignal none of switches 72, 74, 76, 78, 80 and 82 are operational (e.g.,the switches are electrically locked out) until the multiphase controlmodule 98 receives an input indicating that the problem was cleared, forexample, a user provides such an indication via the user interface 60.Such an approach assures that a backfeed will not occur as a result of aswitch malfunction by preventing, for example, a connection between thefirst terminal 90 of the primary input 91 and the second terminal 108 ofthe alternate input 93 via the load connected to the output 84.

The multiphase control module 98 generates a logic signal to operateswitches 72, 74 from their normally open (NO) position to their normallyclosed (NC) position if switches 76, 78 have successfully switched totheir normally closed (NC) position (Stage 542). After generating thelogic signal to operate switches 72, 74, the multiphase control module98 senses the status of switches 72, 74 based on the state of logicpoints SWPR1 and SWPR2 (Stage 544). A voltage present at sensing nodes110, 112 indicates that the switches (72, 74, respectively) have movedto their normally closed (NC) position. Thus, if both SWPR1 and SWPR2are LO then switches 110, 112 have in fact moved to their normallyclosed (NC) position, and the multiphase control module 98 will generatea logic signal to operate switches 80, 82 to move them from theirnormally open (NO) position to their normally closed (NC) position(Stage 546). If, however, either SWPR1 or SWPR2 are HI then thecorresponding switch (72, 74, respectively) has malfunctioned andtherefore failed to move to the normally closed (NC) position. As aresult, the multiphase control module 98 senses a switch malfunction andgenerates a signal (e.g., a fault signal, trouble signal, etc.)indicating that there is fault with either or both of switches 72, 74(Stage 424).

The multiphase control module 98 generates logic signals to operateswitches 80, 82 from their normally open (NO) position to their normallyclosed (NC) position if switches 72, 74 have successfully switched totheir normally closed (NC) position (Stage 546). After generating thelogic signal to operate switches 80, 82, the multiphase control module98 senses the status of switches 80, 82 based on the state of logicpoint CKTO (Stage 548). When voltage is present at the first terminal 86of the output 84 and the second terminal of the output 88, CKTO willprovide a LO signal indicating that switches 80, 82 have successfullymoved to their normally closed (NC) position. Conversely, with CKTO HIthe multiphase control module 98 senses a malfunction of at least one ofthe switches 80, 82 and generates a signal (e.g., a fault signal,trouble signal, etc.) indicating that there is fault with either or bothof switches 80, 82 (Stage 424). The primary input 92 remains connectedto the output 84 provided that switches 80, 82 successfully switched totheir normally closed (NC) position and provided that multiphase controlmodule 98 does not initiate a transfer to an alternate source, forexample, if the primary power source fails.

The embodiment described with reference to FIGS. 3, 4 and 5 employs themultiphase control module 98 to detect the availability of the primaryand the alternate sources, i.e., via PR1/PR2 and ALT1/ALT2,respectively. In another embodiment, however, the main control module 58is employed to monitor logic points PR1, PR2, ALT1, and ALT2. In aversion of this embodiment, the multiphase control module 98 waits toreceive information from the main control module 58 concerning thestatus of the primary and the alternate sources of power beforeperforming any switching.

In one embodiment, the logic described with reference to FIGS. 3, 4 and5 is implemented in a state machine implemented by the multiphasecontrol module 98. In a version of this embodiment, a CPLD includes thestate machine. Further, the state machine can be embodied in analgorithm or a plurality of algorithms stored in the memory and executedby a processor or processors located in the multiphase control module98. In one embodiment, the memory is included in the multiphase controlmodule 98. In a version of the preceding embodiments, a portion of thelogic described with reference to FIGS. 3, 4 and 5 is implemented in themultiphase control module 98 and another portion of the logic isimplemented in the main control module 58.

Each of the switching modules described thus far can also includeintegral overcurrent protection. For example, in embodiments of each ofthe switching modules described herein (e.g., 52, 54, 70, etc.), a fusemay be included in each line conductor. In a version of this embodiment,the fuse is connected to the output of the switching module with whichit is associated (e.g., each line of output 84 of FIG. 2). In addition,embodiments of each of the switching modules described herein (e.g., 52,54, 70, etc.) may include current and voltage sensing. For example, inan embodiment of multiphase switching modules 70, 70′ of FIG. 2, a firstcurrent sensor senses current flow at a point between switch 80 andfirst terminal 86, a second current sensor senses current flow at apoint between switch 82 and second terminal 88. In a version of thisembodiment, a primary of a first potential transformer is connected tofirst terminal 86 and second terminal 88 to decrease the voltage levelsupplied to the multiphase control module 222. The outputs of thecurrent sensors and the voltage sensors can be supplied to the maincontrol module 58 for processing, display at the user interface 60, andcommunication to remote systems via the communication module 64 shown inFIG. 1.

FIGS. 6, 7 and 8 depict alternative embodiments of multiphase switchingmodules. Each of the switching modules shown in FIGS. 6, 7 and 8 can beemployed in a transfer switch, for example, the transfer switch 20 shownin FIG. 1, in place of multiphase switching modules 54, 54′.

In FIG. 6, switching modules 121, 121′ are employed to control a twophase electric supply to a load 123. A primary source of power isconnected to a primary input 130 and an alternate source of power isconnected to an alternate input 132. The switching modules 121, 121′also provide an output 133. The primary input 130 includes a firstterminal 122 and a second terminal 124 to connect a first phase and asecond phase of the primary source of power to the multiphase switchingmodules 121, 121′, respectively. The alternate input 132 includes afirst terminal 126 and a second terminal 128 to connect a first phaseand a second phase of the alternate source of power to the multiphaseswitching modules 121, 121′. In the embodiment shown in FIG. 6, switches134, 136, 138, 140, 142, 144 are included in the multiphase switchingmodules 121, 121′. In another embodiment, however, the switches 134,136, 138, 140, 142, 144 are included in a single multiphase switchingmodule. The approach shown in FIG. 6 is scalable, so that in yet anotherembodiment a multiphase switching module is included to control a thirdphase of a three phase electric supply. In one embodiment, switches 134and 136 are employed to isolate the alternate input 132 from theremainder of the circuitry. The switches of one or more switchingmodules form sets of switches where each set is used to switch allphases of a specific input or output. In FIG. 6, for example, switches134, 136 form a first set of switches that switch the alternate input132, switches 138, 140 form a second set of switches that switch theprimary input 130, and switches 142, 144 form a third set switches thatswitch the output 133. Each set of switches in FIG. 6 include a pair ofswitches, however, each set may include three or more switches dependingupon the requirements of the application, e.g., the quantity of lineconductors provided by the power sources.

The embodiment of FIG. 6 also includes a multiphase control module 146.The multiphase control module 146 includes a first module 148 forsensing and control of a first phase and a second module 150 for sensingand control of a second phase. In alternative embodiments, themultiphase control module 146 may be a single module as shown for theembodiment in FIG. 1. A neutral conductor 152 is also connected tomultiphase control module 146. As described above concerning themultiphase control modules 70, 70′ in FIG. 2, the multiphase controlmodule 146 of FIG. 6 includes sensing inputs and switching outputs toensure that a backfeed between the primary source and the alternatesource does not occur at any time, even if one or more of the switches134, 136, 138, 140, 142 and 144 malfunction. For example, the sensingand control logic of the multiphase control module 146 prevent abackfeed that can result when a switch becomes inoperative as a resultof a welded contact. In one embodiment, the approach employed in thecontrol of the multiphase switching modules 121, 121′ is generally thesame as that previously described for switching modules 70, 70′.Switching modules 121, 121′ include sensing nodes that provide signalsto the multiphase control module 146 that allow the control module 146to detect one or more inoperative switches among switches 134, 136, 138,140, 142 and 144 and control switch operation to prevent a connectionbetween the primary input 130 and the secondary input 132.

The configuration of switches 134, 136, 138, 140, 142 and 144 in theembodiment shown in FIG. 6 does differ from the configuration ofswitches 72, 74, 76, 78, 80 and 82 in the embodiment shown in FIG. 2.For example, in FIG. 6, the switches 138, 140 used to switch the primaryinput 130 have a terminal connected to the primary input 130 and aterminal connected to the switches 134, 136 used to switch the alternateinput 132, i.e., switch 134 is connected to switch 138 and switch 136 isconnected to switch 140. Conversely, in the embodiment shown in FIG. 2,the switches 72, 74 used to switch the primary input 91 are notconnected to the switches 76, 78 used to switch the alternate input 93.Instead, in the embodiment shown in FIG. 2, switches 80, 82 selectivelyconnect the output 84 to either switches 72, 74, or switches 76, 78.

In FIG. 7, a further embodiment of multiphase switching modules 154,154′, which can be used in place of the multiphase switching modules 54,54′ of FIG. 1 in the transfer switch 20, is shown. The multiphaseswitching modules 154, 154′ include a primary input 157, an alternateinput 158 and an output 159. The multiphase switching modules 154, 154′include a set of switches 160, 162 to switch the primary input 157, aset of switches 164, 166 to switch the alternate input 158, and a set ofswitches 168, 170 to switch the output 159. Switches 160, 164 and 168are included in a first multiphase switching module 154, and switches162, 166 and 170 are included in a second multiphase switching module154′. The switch configuration of the embodiment shown in FIG. 7 is thesame as the switch configuration shown in FIG. 6, however, theembodiment of FIG. 7 differs from the embodiment shown in FIG. 6. As oneexample, the multiphase switching modules 154, 154′ may be connected toa test load 172. In one version, the test load 172 is located externalto the transfer switch 20. In another version, the test load 172 isincluded within the transfer switch 20. In addition, multiphaseswitching modules 154, 154′ employ current sensors 174, 174′ to detectcurrent flow resulting from a malfunction of any of the switches 160,162, 164, 166, 168, 170 employed to control the two phase electricsupply to a load 176. In one embodiment, the current sensors 174, 174′are a torodial style current transformer, for example, part numberL12003 manufactured by Falco Electronics.

In the embodiment shown in FIG. 7, a multiphase control module 178employs logic to prevent a backfeed between any phase of the primaryinput 157 and any phase of the alternate input 158, however, the logicdiffers from that described regarding FIG. 6 because in addition toteachings described above, current sensing may be used with theembodiment of FIG. 7 to detect a malfunction of the switches 168, 170.For example, when the connection to the output 159 is transferred fromthe primary input 157 to the alternate input 158, switches 168, 170operate to disconnect the load and connect the test load 172 to theprimary input 157. Switches 160, 162, 164, 166 are then switched toconnect the alternate input 158 to the test load 172. Provided thatthere are no inoperative switches, there is no current flow in thecurrent transformer secondaries 180, 182 during these switchingoperations. More specifically, in one embodiment, switches 160, 162 areswitched to the position that connects them to switches 164, 166,respectively. The switches 164, 166 are switched to the position thatconnects alternate input 158 to switches 160, 162. If the primary input157 is connected to the alternate input 158 during the precedingswitching operations, for example, as a result of a switch malfunction,any current flowing from an input 157, 158 does not have a return paththat is sensed by the corresponding current transformer. As a result,provided that at least one of the primary input 157 and the alternateinput 158 are energized, current flows in the corresponding currenttransformer secondary 180 or 182. The corresponding current transformeroutput 181, 183 then provides a signal to drive an input of themultiphase control module 178. The multiphase control module 178provides a fault indication and, in one embodiment, prevents anysubsequent switch operation until a user resets the fault indication(e.g., after investigating and clearing the fault condition, forexample, replacing a failed switch). If there are no switchmalfunctions, however, switches 168, 170 are operated to connect theoutput 159 to the alternate input 158. Again, provided that there are noinoperative switches, there should be no current flow in the currenttransformer secondaries 180, 182.

In one embodiment, the current drawn by the test load 172 issufficiently small that it is not considered a hazard if a backfeedbetween the primary input 157 and the alternate input 158 is createdthrough the test load 172. In one embodiment, a current is considerednon-hazardous if it is less than 3.5 milliamperes. In anotherembodiment, a lower voltage source (e.g., a DC source or an AC sourcesupplied from a step-down transformer) is connected to the unusedterminal of each of switches 164, 166 to provide a means of providing alow power test current.

FIG. 8 shows a single multiphase switching module 185 which can beemployed, for example, in the transfer switch 20 shown in FIG. 1 inplace of both the multiphase switching modules 54, 54′. The embodimentof FIG. 8 includes a primary input 184, an alternate input 186, anoutput 188, switches 190, 192, 194, 196, 198, 200, and sensing nodes202, 204, 206, 208, 210, 212, 214, 216, 218. The switch configuration ofthe embodiment shown in FIG. 8 is similar to the switch configurationshow in FIG. 7, however, in FIG. 8 the test load 172 of FIG. 7 isreplaced with a current source 220. In addition, current sensors are notincluded at the primary input and the secondary input in the embodimentshown in FIG. 8. In one embodiment (described below), however, currentsensing is employed to determine the status of switches 198, 200, inparticular, sensing current flow from the current source 220.

In one embodiment, each of the sensing nodes 202, 204, 206, 208, 210,212, 214, 216, 218 is connected to a multiphase control module 222 overcommunication bus 224. As previously described with reference to FIG. 2,switch control signals (not shown) control operation of the switches190, 192, 194, 196, 198, 200. In one embodiment, the switch controlsignals are provided to the multiphase switching module 185 by themultiphase control module 222 over the communication bus 224. Logic forthe switching module 185 is implemented in a fashion very similar tothat described for one embodiment of the multiphase switching modules70, 70′ of FIG. 2. For example, in one embodiment, the status ofswitches 190, 192, 194 and 196 is determined by voltage sensing. Thestatus of switches 198, 200 connected to the output 188, however, isdetermined by the current flow in the circuit created by the currentsource 220, switches 198, 200 in their normally closed (NC) position,and a load 226 connected to the output 188. Thus, during normaloperation, current from the current source 220 flows through the load226 and the switches 198, 200 when the output 188 is isolated from theswitches 190, 192.

In the embodiment shown in FIG. 8, with the switches 198, 200 in theirnormally open (NO) position, the transfer from the first input 184 tothe second input 186 begins when the switches 198, 200 are disconnectedfrom the load 226 and connected to the current source 220. If theswitches 198, 200 operate properly, current flows from the currentsource 220 through the load 226. The presence of current is determinedwhen logic associated with sensing node 214 provides a HI signalindicating that switches 198, 200 are in their normally closed position.The multiphase control module 222 detects a malfunction of at least oneof switches 198 and 200 when sensing node 214 provides a logic LO signalindicating that current is not flowing as expected. The multiphasecontrol module 222 provides a fault indication and, in one embodiment,prevents any subsequent switch operation until a user resets the faultindication (e.g., after investigating and clearing the fault condition,for example, replacing a failed switch). If switches 198, 200 operate asexpected, the switches 190, 192 are switched to their normally open (NO)position. The switches 194, 196 are switched to their normally open (NO)position. Voltage sensing at sensing nodes 210, 212 is used to determinewhether switches 194 and 196 have operated properly. In one embodiment,the presence of voltage at sensing nodes 210, 212 indicates that heswitches 194, 196 operated properly. The switches 198, 200 are thenswitched to their normally open (NO) position to connect the alternateinput 186 to the output 188. If the switches 198, 200 operated properly,the logic associated with sensing node 214 transitions to a LO signalindicating that current from current source 220 is no longer flowingthrough switches 198, 200.

In one embodiment, multiphase switching module 185 includes twomultiphase switching modules (as shown in the previous embodiments) withthe current source 220 located external to each switching module.

Several aspects of at least one embodiment of this invention have beendescribed herein with reference to 240 volt systems. The embodimentsdescribed herein, however, may also be employed with a wide range ofsystems including three phase systems and systems operating at voltagessuch as, for example, 480 volt systems. Further, the embodiments of theinvention may be employed in AC and DC systems including AC systemsoperating at 50 Hz, 60 Hz or other frequencies.

Embodiments of transfer switch 20 described herein can also beconfigured to selectively switch between three or more multiphasesources of power, for example, a primary power source, a first alternatepower source, and a second alternate power source. In these embodiments,the multiphase switching modules (e.g., 70, 70′) are configured toswitch between the multiple sources, i.e., the embodiments describedherein are scalable. In versions of these embodiments, the transferswitch complies with UL 1008 and prevents a backfeed even when only asingle switch fails.

Further, embodiments of the invention including those described hereinmay be employed in a variety of transfer switches. For example,embodiments of the invention may be employed in an automatic transferswitch that switches between a primary source and an alternate sourcewithout user intervention. Versions of these embodiments may alsoprovide a start signal to automatically start a generator. Embodimentsof the invention may also be employed in transfer switches that switchbetween a primary power source and an alternate power source wheninstructed to do so by a user. In these embodiments, the multiphasecontrol module can employ logic as described herein to prevent abackfeed between two or more sources of power.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art. Such alterations, modifications, and improvements are intendedto be part of this disclosure, and are intended to be within the spiritand scope of the invention. Accordingly, the foregoing description anddrawings are by way of example only.

What is claimed is:
 1. A transfer switch comprising: a first input tocouple a first multiphase low voltage electrical supply to the transferswitch; a first set of switches in electrical communication with thefirst input; a second input to couple a second multiphase low voltageelectrical supply to the transfer switch; a second set of switches inelectrical communication with the second input; an output to couple thetransfer switch to a load; a third set of switches coupled in seriesbetween the second set of switches and the output; and a control moduleto monitor and control operation of the first set of switches, thesecond set of switches and the third set of switches to selectivelycouple the first input and the second input to the output and to detecta malfunction of any switches included in at least one of the first setof switches, the second set of switches and the third set of switchesand to prevent the first input from being placed in electricalcommunication with the second input, wherein the second set of switchesare coupled in series between the first set of switches and the thirdset of switches.
 2. The transfer switch as claimed in claim 1, furthercomprising a switching module comprising: a switch included in the firstset of switches; a switch included in the second set of switches; and aswitch included in the third set of switches.
 3. The transfer switch asclaimed in claim 2, wherein the switching module further comprises: eachswitch included in the first set of switches; each switch included inthe second set of switches; and each switch included in the third set ofswitches.
 4. The transfer switch as claimed in claim 2, furthercomprising a plurality of switching modules.
 5. The transfer switch asclaimed in claim 1, wherein the first input is adapted to couple to afirst split-phase electrical supply, and wherein the second input isadapted to couple to a second split-phase electrical supply.
 6. Thetransfer switch as claimed in claim 5, wherein the first input isadapted to couple to a first 240 volt split-phase electrical supply, andwherein the second input is adapted to couple to a second 240 voltsplit-phase electrical supply.
 7. The transfer switch as claimed inclaim 6, wherein at least one of the first input and the second input isadapted to couple to a power source selected from a group consisting ofa generator, an uninterruptible power supply and an alternative energysource.
 8. The transfer switch as claimed in claim 1, wherein the firstset of switches comprises a first pair of switches and the second set ofswitches comprises a second pair of switches.
 9. The transfer switch asclaimed in claim 8, the first set of switches further comprising twosingle pole double throw switches, and the second set of switchesfurther comprising two single pole double throw switches.
 10. Thetransfer switch as claimed in claim 9, the first set of switches furthercomprising a first relay comprising at least one single pole doublethrow switch; and the second set of switches further comprising a secondrelay comprising at least one single pole double throw switch.
 11. Thetransfer switch as claimed in claim 8, wherein the first pair ofswitches comprises a first switch and a second switch, wherein thecontrol module is coupled to a load side of the first switch to receivea first control signal, wherein the control module is coupled to a loadside of the second switch to receive a second control signal, andwherein the control module is adapted to detect a malfunction of thefirst set of switches by a state of at least one of the first controlsignal and the second control signal.
 12. The transfer switch as claimedin claim 11, wherein the second pair of switches comprises a firstswitch and a second switch, wherein the control module is coupled to aload side of the first switch of the second pair of switches to receivea third control signal, wherein the control module is coupled to a loadside of the second switch of the second pair of switches to receive afourth control signal, and wherein the control module is adapted todetect a malfunction of the second set of switches by a state of atleast one of the third control signal and the fourth control signal. 13.The transfer switch as claimed in claim 1, wherein a malfunction of thethird set of switches is sensed by a presence of voltage at the output.14. The transfer switch as claimed in claim 1, wherein a malfunction ofthe third set of switches is sensed by current flow through any of theswitches comprising the third set of switches.
 15. The transfer switchas claimed in claim 1, wherein the malfunction includes a weldedcontact.
 16. The transfer switch as claimed in claim 1, wherein thethird set of switches are operable in at least a first position and asecond position, wherein the second set of switches are coupled to theoutput when the third set of switches are located in the first position,and wherein the second set of switches are coupled to a test load whenthe third set of switches are located in the second position.
 17. Thetransfer switch as claimed in claim 1, wherein the third set of switchesare operable in at least a first position and a second position, whereinthe second set of switches are coupled to the output when the third setof switches are located in the first position, wherein the second set ofswitches are disconnected from the output when the third set of switchesare located in the second position, and wherein the output is coupled toa current source when the third set of switches are located in thesecond position.
 18. The transfer switch as claimed in claim 1, whereinthe second set of switches are operable in at least a first position anda second position, wherein the first set of switches are electricallyisolated from the third set of switches by the second set of switcheswhen the second set of switches are located in the first position, andwherein the first set of switches are placed in electrical communicationwith the third set of switches by the second set of switches when thesecond set of switches are located in the second position.
 19. A methodof preventing a backfeed through a transfer switch, the transfer switchcomprising a first input coupled to a first switch and adapted toreceive a first multiphase low voltage power from a first power source,a second input coupled to a second switch and adapted to receive asecond multiphase low voltage power from a second power source, anoutput and a third switch coupled to the output, the method comprising:connecting the first input to the output at least in part by operatingthe second switch to connect the first switch to the third switch;initiating a transfer to disconnect the output from the first input andconnect the output to the second input at least in part by operating thesecond switch to connect the second input to the third switch; operatinga plurality of sets of switches to complete the transfer; monitoring atleast one of the plurality of sets of switches to detect a malfunctionof any switches; and stopping the transfer if a malfunction is detectedto prevent the first input from being placed in electrical communicationwith the second input.
 20. The method of claim 19, wherein the act ofinitiating comprises an act of detecting a voltage decrease at the firstinput.
 21. The method of claim 19, wherein the act of monitoringcomprises an act of voltage sensing.
 22. The method of claim 21, furthercomprising acts of: sensing whether the first switch is operative bysensing a voltage at a load side of the first switch; sensing whetherthe second switch is operative by sensing a voltage at a load side ofthe second switch; and sensing whether the third switch is operative bysensing a voltage at a load side of the third switch.
 23. The method ofclaim 22, wherein each of the acts of sensing further comprise an act ofdetecting a welded contact by the presence of a voltage.
 24. The methodof claim 19, further comprising an act of sensing whether a switch is ina position that is inconsistent with a state established for the switchby control logic.
 25. The method of claim 19, wherein the first input isadapted to couple to a first split-phase low voltage power source andthe second input is adapted to couple to a second split-phase lowvoltage power source.
 26. The method of claim 19, wherein each of theplurality of sets of switches comprises a pair of switches.
 27. Themethod of claim 19, further comprising an act of sensing that a switchconnected to the output is inoperative when a current is flowing throughthe switch connected to the output.
 28. A transfer switch comprising: afirst input to couple a first multiphase low voltage electrical powersupply to the transfer switch; a first set of switches in electricalcommunication with the first input; a second input to couple a secondmultiphase low voltage electrical power supply to the transfer switch; asecond set of switches in electrical communication with the secondinput; an output to couple the transfer switch to a load; a third set ofswitches coupled in series between the second set of switches and theoutput; and means for detecting a malfunction in any of the first set ofswitches, the second set of switches and the third set of switches, andupon detecting a malfunction, preventing a connection of at least one ofthe first input and the second input to the output, wherein the secondset of switches are coupled in series between the first set of switchesand the third set of switches.
 29. The transfer switch as claimed inclaim 28, further comprising a switching module comprising: a switchincluded in the first set of switches; a switch included in the secondset of switches; and a switch included in the third set of switches. 30.The transfer switch as claimed in claim 29, further comprising aplurality of switching modules.
 31. The transfer switch as claimed inclaim 28, wherein the means for detecting a malfunction detects amalfunction of a switch in the first set of switches when a voltage ispresent on a load side of the switch.
 32. The transfer switch as claimedin claim 28, wherein the means for detecting a malfunction detects amalfunction of a switch in the second set of switches when a voltage ispresent on a load side of the switch.
 33. The transfer switch as claimedin claim 28, wherein the first input is adapted to couple to a firstsplit-phase low voltage power supply and the second input is adapted tocouple to a second split-phase low voltage power supply.
 34. Thetransfer switch as claimed in claim 33, wherein the first input isadapted to couple to a first 240 volt split-phase power supply and thesecond input is adapted to couple to a second 240 volt split-phase powersupply.
 35. The transfer switch as claimed in claim 28, wherein thesecond input is adapted to couple to a power supply selected from agroup consisting of a utility source, a generator, an uninterruptiblepower supply and an alternative energy source.
 36. The transfer switchas claimed in claim 28, wherein, upon detecting a malfunction, the meansfor detecting a malfunction generate a fault indication.
 37. Thetransfer switch as claimed in claim 28, wherein each of the first set ofswitches and the second set of switches comprises a pair of switches.38. A transfer switch comprising: a first input to couple to a firstpower source; a second input to couple to a second power source; anoutput to provide output power; a first switching module coupled to thefirst input, the second input and the output and configured toselectively couple one of the first input and the second input with theoutput; a second switching module coupled to the first input, the secondinput and the output and configured to selectively couple one of thefirst input and the second input with the output; an interfaceconfigured to allow a user to select a single phase configuration or amultiphase configuration for each of the first switching module and thesecond switching module; and a control device coupled to the interfaceand the first switching module and the second switching module andconfigured to prevent the first input from being placed in electricalcommunication with the second input for the multiphase configuration ofeither the first switching module or the second switching module. 39.The transfer switch of claim 38, wherein the control device isconfigured to detect a loss of power at the first input and to controleach of the first switching module and the second switching module todisconnect the output from the first input and connect the output to thesecond input while isolating the first input from the second input. 40.The transfer switch of claim 39, wherein the output includes a firstoutput line and a second output line and wherein the first switchingmodule is coupled to the first output line and the second switchingmodule is coupled to the second output line.
 41. The transfer switch ofclaim 38, wherein each of the first input and the second input includesa first line and a second line, and in the multiphase configuration, thefirst line of the first input and the first line of the second input arecoupled to the first switching module and the second line of the firstinput and the second line of the second input are coupled to the secondswitching module.
 42. The transfer switch of claim 41, furthercomprising a third input, a fourth input and a second output and a thirdswitching module coupled to the third input, the fourth input and thesecond output and configured to selectively couple one of the thirdinput and the fourth input with the second output.
 43. The transferswitch of claim 38, wherein the first switching module has at least oneswitch, and wherein the control device is configured to detect a failureof the at least one switch.
 44. A method of switching multiple powersources to a load using a transfer switch having at least a firstswitching module and a second switching module, the method comprising:coupling a first power source to the first switching module and to thesecond switching module; coupling a second power source to the firstswitching module and to the second switching module; configuring a userinterface of the transfer switch to allow a user to select between asingle phase mode of operation and a multiphase mode of operation foreach of the first switching module and the second switching module;monitoring at least one of the first switching module and the secondswitching module to detect a malfunction within the at least one of thefirst switching module and the second switching module; and preventing atransfer of power from the first power source to the second power sourceupon detection of a malfunction.
 45. The method of claim 44, furthercomprising coupling a first load to an output of the first switchingmodule and to an output of the second switching module and providing 240volt AC power to the first load.
 46. The method of claim 45, furthercomprising coupling a third power source to a third switching module ofthe transfer switch.
 47. The method of claim 46, further comprisingcoupling a second load to an output of the third switching module andproviding 120 volt single phase power to the second load.
 48. The methodof claim 44, further comprising selecting a single phase mode ofoperation for each of the first switching module and the secondswitching module, and coupling a first load to the first switchingmodule and coupling a second load to the second switching module andproviding 120 volt single phase power to the first load and to thesecond load.
 49. The method of claim 44, wherein: the first power sourceand the second power source are split-phase power sources; coupling afirst power source to the first switching module and to the secondswitching module includes coupling the first switching module to a firstphase line of the first power source and coupling the second switchingmodule to a second phase line of the first power source; and coupling asecond power source to the first switching module and to the secondswitching module includes coupling the first switching module to a firstphase line of the second power source and coupling the second switchingmodule to a second phase line of the second power source.
 50. A transferswitch comprising: a first input to couple to a first power source; asecond input to couple to a second power source; an output to provideoutput power; a first switching module coupled to the first input, thesecond input and the output and configured to selectively couple one ofthe first input and the second input with the output; a second switchingmodule coupled to the first input, the second input and the output andconfigured to selectively couple one of the first input and the secondinput with the output; means tor selecting a single phase configurationor a multiphase configuration for each of the first switching module andthe second switching module and means for preventing the first inputfrom being placed in electrical communication with the second input forthe multiphase configuration.
 51. The transfer switch of claim 50,further comprising means for detecting a loss of power at the firstinput and for controlling each of the first switching module and thesecond switching module to disconnect the output from the first inputand connect the output to the second input while isolating the firstinput from the second input.
 52. The transfer switch of claim 50,wherein the output includes a first output line and a second output lineand wherein the first switching module is coupled to the first outputline and the second switching module is coupled to the second outputline.
 53. The transfer switch of claim 50, wherein each of the firstinput and the second input includes a first line and a second line, andin the multiphase configuration, the first line of the first input andthe first line of the second input are coupled to the first switchingmodule and the second line of the first input and the second line of thesecond input are coupled to the second switching module.
 54. Thetransfer switch of claim 53, further comprising a third input, a fourthinput and a second output and a third switching module coupled to thethird input, the fourth input and the second output and configured toselectively couple one of the third input and the fourth input with thesecond output.
 55. The transfer switch of claim 50, wherein the firstswitching module has at least one switch, and wherein the transferswitch further includes means for detecting a failure of the at leastone switch.